The present invention relates to a novel method for promoting liver regeneration, and in particular to a method using a composition featuring the IL-6/sIL-6R complex for promoting liver cell proliferation and liver weight restoration, as well as the restoration of liver functions.
The loss of liver functions from traumatic or toxic injury or disease may cause severe debilitation or even death. There are many causes for the loss of liver functions, including malignancies in the liver, both primary and those which metastasize to the liver from another location in the body, viral diseases such as the many forms of viral Hepatitis, and hepatotoxicity caused by exposure to excessive liver toxins such as drug overdose and pesticides. Indeed, even some normally non-toxic substances can become hepatotoxic when abused, such as paracetamol and ethanol. Thus, injury to the liver, resulting in the loss of liver functions, can have many different initial causes.
Patients with acute liver failure have high morbidity and mortality rates. Only 40% of patients treated with conservative medical treatment alone survive. Those patients which do survive are somehow able to restore liver functions. Among the essential functions of the liver are glucose regulation, synthesis of many blood proteins like albumin and coagulation proteins, secretion of bile, biodegradation of toxic compounds, and others.sup.1 (see Appendix for a complete list of references). Little if any disturbance is observed in these functions when only 33% of the liver remains intact and 90% of the remaining cells undergo proliferation and regeneration.sup.1.
Liver regeneration is important for the restoration of liver functions in response to injury, either disease or trauma induced. The term "liver regeneration" is defined as an orchestrated response induced by specific external stimuli and involving sequential changes in gene expression, growth factor production, and morphological structure.sup.1. Studies have shown that when rats are joined in pairs through parabiotic circulation, hepatectomy of one member of the pair causes regeneration of the intact liver of the other member, with the maximum effect seen when the liver of one animal is totally removed.sup.2,3. As demonstrated by these and other studies, many soluble factors, such as multiple growth factors and cytokines, are mitogenic signals for hepatocytes during liver regeneration.
Several lines of evidence suggest that TNF-.alpha. (Tumor necrosis factor alpha) and IL-6 (Interleukin-6) are the most crucial components of the early signaling pathways leading to regeneration. IL-6 is secreted by Kupffer cells, and this secretion is stimulated by TNF-.alpha.. IL-6 is an important signal for the initiation of acute phase protein synthesis by hepatocytes as a part of the overall inflammatory response.sup.4. Recent experiments have demonstrated that liver regeneration following partial hepatectomy (pHx) is massively impaired in mice carrying a homozygous deletion of the IL-6 gene.sup.8 or of the TNF-.alpha. type I receptor gene.sup.9. Furthermore, the plasma IL-6 concentration increases after pHx (partial hepatectomy), reaching high levels by 24 hours after the removal of liver tissue.sup.5-7. Thus, IL-6 and TNF-.alpha. are important components of the response to liver injury.
On target cells, IL-6 first binds to a specific IL-6 receptor.sup.12. This IL-6/sIL-6R complex induces the homodimerization of two gp130 signal transducing molecules.sup.13, 14 leading to intracellular signaling events. Soluble forms of the IL-6R (sIL-6R) are generated by limited proteolysis from the cell surface.sup.15 and render cells which do not express membrane bound IL-6R responsive towards IL-6.sup.16. Furthermore, sIL-6R acts as a serum-binding protein for IL-6 and prolongs the plasma half-life of IL-6.sup.17. The presence of the IL6/sIL-6R complex in IL-6/sIL-6R double transgenic mice leads to a marked extramedullary expansion of hematopoietic progenitor cells.sup.18. The presence of IL-6 alone does not cause similar effects. Thus, the IL-6/sIL-6R appears to have certain effects which extend beyond those of IL-6 alone.
Unfortunately, in spite of many published findings regarding the effects of IL-6 and other molecular components of the liver regeneration pathway, few suitable treatments are available for those suffering from a loss of liver functions. Liver transplantation is the only established therapy which has been shown to improve the survival rate of patients with acute liver failure. However, transplantation is a time consuming and costly therapy associated with a life-long requirement for immunosuppression. Long-term side-effects of the immunosuppression remain unevaluated. In addition, as for any type of organ transplantation, suitable donors are not always available.
Therapies based on the molecular basis of liver regeneration have been examined in an attempt to the development of new treatment strategies beyond liver transplantation. For example, experiments in which recombinant HGF (hepatocyte growth factor) was administered to animals following liver injury suggested that the administration of HGF might be beneficial in liver regeneration and that HGF might help to improve the regenerating capacity of the liver.sup.29-33. However, since the plasma half-life of HGF is extremely short (t 1/2 of 4.5 min), HGF must be administered by a continuous infusion into a peripheral vein or into the portal vein. Such inconveniently frequent administration is a severe drawback to HGF treatment.
There is therefore a need for, and it would be useful to have, a novel treatment for liver injury which would promote the restoration of liver functions in a subject by promoting and enhancing liver regeneration.